add unit test

Author: Lms24

packages/browser/src/tracing/browserTracingIntegration.ts

@@ -284,24 +284,24 @@ export const browserTracingIntegration = ((_options: Partial<BrowserTracingOptio
         });
       });
 
+      // A trace should to stay the consistent over the entire time span of one route.
+      // Therefore, when the initial pageload or navigation transaction ends, we update the
+      // scope's propagation context to keep span-specific attributes like the `sampled` decision and
+      // the dynamic sampling context valid, even after the transaction has ended.
+      // This ensures that the trace data is consistent for the entire duration of the route.
       client.on('spanEnd', span => {
         const op = spanToJSON(span).op;
         if (span !== getRootSpan(span) || (op !== 'navigation' && op !== 'pageload')) {
           return;
         }
 
         const scope = getCurrentScope();
+        const oldPropagationContext = scope.getPropagationContext();
 
-        // A trace should to stay the same for the entire time span of one route.
-        // Therefore, when the initial pageload or navigation transaction is ended, we update the
-        // scope's propagation context to keep span-specific attributes like the `sampled` decision and
-        // the dynamic sampling context valid, even after the transaction has ended.
-        // This ensures that the trace data is consistent for the entire duration of the route.
         const newPropagationContext = {
-          ...scope.getPropagationContext(),
-          // it's okay to always set sampled: true/false here. If we have a span, it cannot be un-sampled.
-          sampled: spanIsSampled(span),
-          dsc: getDynamicSamplingContextFromSpan(span),
+          ...oldPropagationContext,
+          sampled: oldPropagationContext.sampled !== undefined ? oldPropagationContext.sampled : spanIsSampled(span),
+          dsc: oldPropagationContext.dsc || getDynamicSamplingContextFromSpan(span),
         };
 
         scope.setPropagationContext(newPropagationContext);

packages/browser/test/unit/tracing/browserTracingIntegration.test.ts

@@ -638,7 +638,7 @@ describe('browserTracingIntegration', () => {
       expect(getCurrentScope().getScopeData().transactionName).toBe('test navigation span');
     });
 
-    it("resets the scopes' propagationContexts", () => {
+    it("updates the scopes' propagationContexts on a navigation", () => {
       const client = new BrowserClient(
         getDefaultBrowserClientOptions({
           integrations: [browserTracingIntegration()],
@@ -675,6 +675,45 @@ describe('browserTracingIntegration', () => {
       expect(newIsolationScopePropCtx?.traceId).not.toEqual(oldIsolationScopePropCtx?.traceId);
       expect(newCurrentScopePropCtx?.traceId).not.toEqual(oldCurrentScopePropCtx?.traceId);
     });
+
+    it("saves the span's sampling decision and its DSC on the propagationContext when the span finishes", () => {
+      const client = new BrowserClient(
+        getDefaultBrowserClientOptions({
+          tracesSampleRate: 1,
+          integrations: [browserTracingIntegration({ instrumentPageLoad: false })],
+        }),
+      );
+      setCurrentClient(client);
+      client.init();
+
+      const navigationSpan = startBrowserTracingNavigationSpan(client, {
+        name: 'mySpan',
+        attributes: { [SEMANTIC_ATTRIBUTE_SENTRY_SOURCE]: 'route' },
+      });
+
+      const propCtxBeforeEnd = getCurrentScope().getPropagationContext();
+      expect(propCtxBeforeEnd).toStrictEqual({
+        spanId: expect.stringMatching(/[a-f0-9]{16}/),
+        traceId: expect.stringMatching(/[a-f0-9]{32}/),
+      });
+
+      navigationSpan!.end();
+
+      const propCtxAfterEnd = getCurrentScope().getPropagationContext();
+      expect(propCtxAfterEnd).toStrictEqual({
+        spanId: propCtxBeforeEnd?.spanId,
+        traceId: propCtxBeforeEnd?.traceId,
+        sampled: true,
+        dsc: {
+          environment: 'production',
+          public_key: 'examplePublicKey',
+          sample_rate: '1',
+          sampled: 'true',
+          transaction: 'mySpan',
+          trace_id: propCtxBeforeEnd?.traceId,
+        },
+      });
+    });
   });
 
   describe('using the <meta> tag data', () => {